There may be reasons why one does not want to forgive another, but the one main and good reason to consider forgiving someone for whatever they have done is simply for the self to move forward and grow. It's true that you will never forget at first and gaining the strength to forgive is sometimes a battle, but healing with Truth through your heart will help to elevate your spirits of the soul to move up in self-worth level and simply forgive.
Forgiveness is not a destiny, it is a choice. Do you want to feel hurt every time you think about the past? Or regret why things ended up the way they did? Was it your fault, the other persons, or a little of both that either may not have admitted to? When you start to discover forgiveness it's the Big Picture that needs to be focused on not the tiny details of the matter. All that starts to matter is the well being of "you" moving into your highest good. Then secondly, it's moving the situation into the good direction. No one wants to remember or regret, and only when there is full forgiveness you will forget and live as a "lighter" person.
Later, the well being of the other person will come through, as soon as they start to realize their mistakes and recap the truth because they saw that you started to. There will be those times when the other person does not realize soon enough about the "illness on their side", but that is not of your concern that is of theirs. It lies with them, not you. Remember you just need to worry about you first and taken the steps to self-heal. Your reactions should not depend on the other persons actions, if you can help it.
Some ways to help you through finding forgiveness because you feel you should make peace with the other person or the situation itself are ways such as meditation, getting enough sleep, eating a diet filled with soul food, and last but not least physical exercise. There are many sources out there in this world to help you with balance and self-grounding. Start to listen to your intuition and take action to know which one best fits you. If you don't know the best one that fits you, it is usually the source that you experience or stumble upon that makes you have good feelings about yourself and feel good afterward. Be open and surrender to the healing yourself and forgiveness will be in your heart sooner than later. At the end of the day the real question is, "Why did this happen?" and "What life value am I to learn from this matter?"
"If we forgive sooner, adapt and change for goodness with less time, we grow and move forward faster."~PaLiChi
As a self-grounded healer I am here to help grow your self-consciousness with Truth to the good direction and into your highest good.